Ethanol is known to expand and increase the thermal motion, commonly termed the "membrane fluidity" of some biological and artificial membranes. Changes in the membrane fluidity can affect such biological functions as membrane permeability, transport and enzymatic activity. Ethanol is also thought to affect the nervous system by a direct action on excitable membranes. In a recent study on artificial membrane vesicles, the lipid fluidity has been found to change with transmembrane potential. The objective of this project is to investigate the in vitro effects of ethanol on the membrane fluidity as well as membrane potential of rat brain synaptosomes. The change in the membrane fluidity was measured by the method of fluorescence depolarization with 1,6 diphenyl, 1,3,5-hexatriene (DPH) as a probe. Membrane potential was monitored by the fluorescence intensity of potential sensitive dye Rhodamine 6G. The fluorescence polarization of DPH incorporated into the synaptosomes decreases with increasing concentrations of ethanol (0-500 mM). The calculated rotational rates are higher in the presence of ethanol indicating the increase in membrane fluidity caused by ethanol. The fluorescence intensity of voltage sensitive dye Rhodamine 6G in synaptosomes increases upon depolarizing the membrane with elevated extracellular potassium. Upon addition of ethanol to intact synaptosomes (suspended in low potassium medium) the fluorescence intensity enhances indicating the membrane depolarization. In membranes (i) suspended in high potassium medium, (ii) lysed, (iii) 12 hour old, no effect of ethanol is observed.